Here I've illustrated a less familiar seaming method for invisible joining of garter or reverse stockinette panels or sections, also illustrated in Essential Techniques. The most wonderful finishing techniques I've ever found, or developed, are presented in Essential Techniques----the perfect finishing book!

This method, like the mattress stitch seam, is seamed on the facing side of the knitting. To execute, join a bottom loop of a row to a top loop of the corresponding row. On one side it will be necessary to catch a loop that is not quite on the edge of the panel; maintain consistency. Refer to the illustration. Unlike the illustration, the seam must be pulled together as it's worked.

Tip regarding mattress seaming: I've noticed that illustrations of mattress seaming on side seams show the sewing being done on the inside of the edge stitches. For less bulk and greater simplicity, the sewing can be done on the outer half of the edge stitches instead. I've illustrated this in my publication Essential Techniques, and much prefer it this way for both execution and results.